But as Democrat Martha Coakley and Republican Scott Brown head into the rare mid-January race, just who will show up at the polls remains something of a mystery.

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“Conventional wisdom is, the high turnout favors the Democrat. But in a special election, high turnout is volatile because you don’t know who’s showing up,” said Scott Ferson, a Democratic strategist in the state.

Making the vote even tougher to predict: the weather. Tuesday’s forecast calls for a mix of snow and rain, with temperatures in the low 30s.

To make some sense of the early returns, POLITICO put together a guide of five things to watch as the results come in, after polls close at 8 p.m.

Look at the South Shore

It’s no coincidence that Brown’s campaign imagery has highlighted his Everyman appeal. An early ad showed him riding his pickup truck across the state, he’s spent time campaigning in the cold outside Fenway Park, and his “People’s Tour” rally featured New England sports heroes such as Red Sox great Curt Schilling and former Boston College football star Doug Flutie.

There couldn’t be a greater contrast with Coakley, who often has exuded an entitled image on the campaign trail, no more clearly illustrated than when she expressed her preference, in an interview with the Boston Globe, for meeting with party leaders instead of glad-handing supporters. (A telling statistic: Brown has made 66 campaign stops since the primary, while Coakley has made only 19, as of Sunday.) Her apparent confusion about Schilling — in a radio interview, she mistakenly said he was a Yankees fan — only fueled voters’ perception of her as out of touch.

Brown’s narrative has clearly hit a chord with working-class voters, who normally vote Democratic but sound receptive to the Republican’s message in a state where Democrats control all facets of government and the economy is sputtering.

Even though organized labor has lined up behind Coakley’s campaign, a Suffolk University poll conducted last week showed Brown leading Coakley 53 percent to 45 percent among voters in union households. The big test for Brown is whether he can hold onto the blue-collar crowd and mobilize them to the polls.

In a late play for this demographic, Coakley’s campaign and its allies launched an attack portraying Brown as a Wall Street Republican for opposing Obama’s proposed tax on large banks. The Coakley campaign also scrambled to put out a TV ad one day before the election showing her politicking with union members at an American Legion hall.

To get a sense of whether Brown is winning the working-class vote, look no further than early returns from the heavily Democratic Irish Catholic South Shore, where Obama suffered significant drop-offs from John Kerry’s vote in 2004. Brown needs to carry the towns of Braintree and Weymouth, two Democratic strongholds that Obama barely held in 2008.